We aim to understand the molecular basis for light activation, recovery and adaptation of visual receptors. Our approach is to determine the intermolecular coupling magnitudes, stoichiometries, speeds and mechanism that permit automatic amplification, shutoff and sensitivity reduction after pulses of activating illumination of rod outer segment (ROS) membrane preparations of several kinds: whole isolated ROS after in vivo activation, broken open after isolation, and reconstituted from purified protein constituents. We use absorption and fluorescence spectroscopy, pH recording of nucleotide hydrolysis, fast membrane-aqueous phase separation, radiolabelled tracer nucleotides, nucleotide HPLC analysis, kinetic and thermodynamic computerized analytic modeling to specify the roles of rhodopsin bleaching, G-proteins, cGMP phosphodiesterase, arrestin, rhodopsin kinase and nucleotide and phosphoryl transfer enzymes in this process. Knowledge of these details is essential to the diagnosis of molecular defects of diseased receptor function and will accelerate progress in understanding design and defects of all hormone- neurotransmitter receptor - G-protein internal messenger coupling processes of both brain and somatic tissue.